Digestive Acts of Sea Cucumbers May Be Dissolving Coral Reefs

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Sea cucumbers secrete acidic compounds that may be weakening the
structure of the coral communities in which they live.

The main component of a coral reef is calcium carbonate (CaCO3),
a mineral that makes up the corals and much of the sand and
rubble around them. Many of the organisms living on the reef
either add to or absorb CaCO3 from the surrounding environment.
In a healthy reef this would be in balance, but when the calcium
carbonate is out of whack, the reef may be unhealthy and could
cease to grow.

The researchers studied a part of Australia's
Great Barrier Reef known as One Tree Reef. In one specific
area, called DK13, they found lots of sea cucumbers. They
collected these squishy animals and studied them in the lab.

Life of a reef

Researchers measure coral reef growth by measuring how much
calcium carbonate is in the surrounding waters. If CaCO3 levels
drop, it means
the reef is growing. The researchers found that the digestive
action of the sea cucumbers increases the CaCO3 levels in the
waters, and may account for about half of the total nighttime
increase in calcium carbonate.

The sea cucumbers survive by scouring the seafloor for food. They
suck in sand and rubble and send it through their digestive
system, filtering out the edible particles and sending the rest
of the rocks and sand on its way. During this process, the sea
cucumbers produce acids to dissolve the carbonate-based sand,
which produces soluble calcium carbonate minerals that are
released into the surrounding waters.

"This CaCO3: You can argue that it is not 'the reef,' as it is
not coral and seems not to be reef-building," study researcher
Kenneth Schneider, of the Carnegie Institution for Science at
Stanford University, told LiveScience. "You can also claim that
the sea cucumber recycles these materials to make them available
for other organisms ... and I will fully agree with you that sea
cucumbers have an ecological role in recycling."

"In a case where the ability of reef organisms to precipitate
CaCO3 will decrease — say, due to ocean acidification or thermal
stress — ... that sand and rubble that the sea cucumbers dissolve
may hurt the reef to a point where a reef is eroding and its
stability weakens," Schneider said in an email.

"The reef is built mainly by corals and calcifying algae, but
between the structure sand and rubble accumulate," he wrote.
"This material fills up gaps in the reef structure and creates
the sand and rubble on the reef floor at the same time it adds
stability to the reef structure."

The calcium carbonate may not be completely a bad thing: It could
help buffer this secluded reef from the increasingly acidic ocean
— "helping to maintain the overall
health of the coral reef," Schneider said. "Although sea
cucumbers may play a part in reef dissolution, they are also an
important part of an incredible marine environment."

The study was published Dec. 23 in the Journal of Geophysical
Research.

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